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An asteroid is a
minor planet A minor planet is an astronomical object in direct orbit around the Sun (or more broadly, any star with a planetary system) that is neither a planet nor exclusively classified as a comet. Before 2006, the International Astronomical Union (IAU) ...
of the
inner Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Sola ...

inner Solar System
. Historically, these terms have been applied to any astronomical object orbiting the
Sun The Sun is the star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many othe ...

Sun
that did not resolve into a disc in a telescope and was not observed to have characteristics of an active
comet A comet is an icy, small Solar System body A small Solar System body (SSSB) is an object in the Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astr ...

comet
such as a
tail The tail is the section at the rear end of certain kinds of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotrop ...

tail
. As minor planets in the outer Solar System were discovered that were found to have volatile-rich surfaces similar to comets, these came to be distinguished from the objects found in the main
asteroid belt The asteroid belt is a torus-shaped region in the Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizin ...

asteroid belt
. The term "asteroid" refers to the minor planets of the inner Solar System, including those co-orbital with
Jupiter Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but ...

Jupiter
. Larger asteroids are often called planetoids.


Overview

Millions of asteroids exist: many are shattered remnants of
planetesimal Planetesimals are solid objects thought to exist in protoplanetary disks and debris disks. Per the Chamberlin–Moulton planetesimal hypothesis, they are believed to form out of cosmic dust grains. Believed to have formed in the Solar System ab ...
s, bodies within the young Sun's
solar nebula The formation and evolution of the Solar System The Solar SystemCapitalization Capitalization ( North American English) or capitalisation ( British English) is writing a word with its first letter as a capital letter (uppercase letter) ...
that never grew large enough to become
planet A planet is an astronomical body orbiting a star or Stellar evolution#Stellar remnants, stellar remnant that is massive enough to be Hydrostatic equilibrium, rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and ...

planet
s. The vast majority of known asteroids orbit within the main asteroid belt located between the orbits of
Mars Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, being larger than only Mercury (planet), Mercury. In English, Mars carries the name of the Mars (mythology), Roman god of war and is often referred to ...

Mars
and Jupiter, or are co-orbital with Jupiter (the
Jupiter trojan The Jupiter trojans, commonly called trojan asteroids or simply trojans, are a large group of asteroid An asteroid is a minor planet of the Solar System#Inner solar system, inner Solar System. Historically, these terms have been applied to ...
s). However, other orbital families exist with significant populations, including the
near-Earth object A near-Earth object (NEO) is any small Solar System body A small Solar System body (SSSB) is an object in the Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body rega ...
s. Individual asteroids are classified by their characteristic spectra, with the majority falling into three main groups: C-type, M-type, and S-type. These were named after and are generally identified with ,
metal A metal (from Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is appro ...

metal
lic, and
silicate In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and the changes they undergo during ...
(stony) compositions, respectively. The sizes of asteroids varies greatly; the largest,
Ceres Ceres most commonly refers to: * Ceres (dwarf planet) Ceres (; minor-planet designation: 1 Ceres) is the smallest recognized dwarf planet, the closest dwarf planet to the Sun, and the List of notable asteroids, largest object in the main astero ...
, is almost across and massive enough to qualify as a
dwarf planet A dwarf planet is a small planetary-mass object that is in direct orbit of the Sun – something smaller than any of the eight classical planets, but still a world in its own right. The prototypical dwarf planet is Pluto. The interest of d ...
. Asteroids are somewhat arbitrarily differentiated from
comet A comet is an icy, small Solar System body A small Solar System body (SSSB) is an object in the Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astr ...

comet
s and
meteoroid A meteoroid () is a small rocky or metallic body in . Meteoroids are significantly smaller than s, and range in size from small grains to one-meter-wide objects. Objects smaller than this are classified as or . Most are fragments from s or as ...
s. In the case of comets, the difference is one of composition: while asteroids are mainly composed of mineral and rock, comets are primarily composed of dust and ice. Furthermore, asteroids formed closer to the sun, preventing the development of cometary ice. The difference between asteroids and meteoroids is mainly one of size: meteoroids have a diameter of one meter or less, whereas asteroids have a diameter of greater than one meter. Finally, meteoroids can be composed of either cometary or asteroidal materials. Only one asteroid,
4 Vesta Vesta (minor-planet designation A formal minor planet designation is, in its final form, a number–name combination given to a minor planet (asteroid, centaur (minor planet), centaur, trans-Neptunian object and dwarf planet but not comet). S ...
, which has a relatively , is normally visible to the naked eye, and this is only in very dark skies when it is favorably positioned. Rarely, small asteroids passing close to Earth may be visible to the naked eye for a short time. , the
Minor Planet Center The Minor Planet Center (MPC) is the official body for observing and reporting on minor planets under the auspices of the International Astronomical Union The International Astronomical Union (IAU; french: link=yes, Union astronomique interna ...
had data on 930,000 minor planets in the inner and outer Solar System, of which about 545,000 had enough information to be given numbered designations. The United Nations declared 30 June as International
Asteroid Day __NOTOC__ Asteroid Day (also known as International Asteroid Day) is an annual global event which is held on the anniversary of the Siberian Tunguska event that took place on June 30, 1908, the most harmful known asteroid An asteroid is a mi ...
to educate the public about asteroids. The date of International Asteroid Day commemorates the anniversary of the
Tunguska asteroid impact over Siberia The Tunguska event was a massive explosion that occurred near the Podkamennaya Tunguska River in Yeniseysk Governorate (now Krasnoyarsk Krai), Russian Empire, Russia, on the morning of 30 June 1908 (adoption of the Gregorian calendar#Adoption in ...
, Russian Federation, on 30 June 1908. In April 2018, the
B612 Foundation The B612 Foundation is a private Nonprofit organization, nonprofit Private foundation (United States), foundation headquartered in Mill Valley, California, United States, dedicated to planetary science and Asteroid impact avoidance, planetary defe ...
reported "It is 100 percent certain we'll be hit y a devastating asteroid but we're not 100 percent sure when." Also in 2018,
physicist A physicist is a scientist A scientist is a person who conducts scientific research The scientific method is an Empirical evidence, empirical method of acquiring knowledge that has characterized the development of science since at leas ...

physicist
Stephen Hawking Stephen William Hawking (8 January 1942 – 14 March 2018) was an English theoretical physics, theoretical physicist, cosmology, cosmologist, and author who, at the time of his death, was director of research at the Centre for Theoretica ...

Stephen Hawking
, in his final book '' Brief Answers to the Big Questions'', considered an asteroid collision to be the biggest threat to the planet. In June 2018, the US
National Science and Technology Council The National Science and Technology Council (NSTC) is a council in the Executive Branch The executive is the branch of government exercising authority in and holding Moral responsibility, responsibility for the governance of a State (polity), s ...
warned that America is unprepared for an asteroid impact event, and has developed and released the ''"National Near-Earth Object Preparedness Strategy Action Plan"'' to better prepare. According to expert testimony in the
United States Congress The United States Congress is the legislature of the federal government of the United States. It is Bicameralism, bicameral, comprising a lower body, the United States House of Representatives, House of Representatives, and an upper body, t ...

United States Congress
in 2013,
NASA The National Aeronautics and Space Administration (NASA; ) is an independent agency A regulatory agency or regulatory authority, is a Public benefit corporation Public-benefit corporation is a term that has different meanings in differen ...

NASA
would require at least five years of preparation before a mission to intercept an asteroid could be launched.


Discovery

The first asteroid to be discovered,
Ceres Ceres most commonly refers to: * Ceres (dwarf planet) Ceres (; minor-planet designation: 1 Ceres) is the smallest recognized dwarf planet, the closest dwarf planet to the Sun, and the List of notable asteroids, largest object in the main astero ...
, was originally considered to be a new planet. This was followed by the discovery of other similar bodies, which, with the equipment of the time, appeared to be points of light, like stars, showing little or no planetary disc, though readily distinguishable from stars due to their apparent motions. This prompted the astronomer
Sir William Herschel Frederick William Herschel (; german: Friedrich Wilhelm Herschel; 15 November 1738 – 25 August 1822) was a German-born British astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific ...

Sir William Herschel
to propose the term "asteroid", coined in Greek as ἀστεροειδής, or ''asteroeidēs'', meaning 'star-like, star-shaped', and derived from the Ancient Greek ''astēr'' 'star, planet'. In the early second half of the nineteenth century, the terms "asteroid" and "planet" (not always qualified as "minor") were still used interchangeably. Discovery timeline: * 10 by 1849 **
1 Ceres Ceres (; minor-planet designation: 1 Ceres) is a dwarf planet in the asteroid belt between the orbits of Mars and Jupiter. Ceres was the first asteroid discovered, on 1 January 1801 by Giuseppe Piazzi at Palermo Astronomical Observatory in Si ...
, 1801 **
2 Pallas Pallas (minor-planet designation A formal minor-planet designation is, in its final form, a number–name combination given to a minor planet (asteroid, centaur (minor planet), centaur, trans-Neptunian object and dwarf planet but not comet) ...

2 Pallas
1802 **
3 Juno 3 Juno is a large asteroid in the asteroid belt. Juno was the third asteroid discovered, in 1804, by German astronomer Karl Ludwig Harding, Karl Harding. It is one of the List of exceptional asteroids, twenty largest asteroids and one of the two l ...
1804 **
4 Vesta Vesta (minor-planet designation A formal minor planet designation is, in its final form, a number–name combination given to a minor planet (asteroid, centaur (minor planet), centaur, trans-Neptunian object and dwarf planet but not comet). S ...
1807 ** 5 Astraea 1845 ** ''in 1846, planet Neptune was discovered'' **
6 Hebe Hebe (minor planet designation A formal minor-planet designation is, in its final form, a number–name combination given to a minor planet A minor planet is an astronomical object in direct orbit around the Sun (or more broadly, any star ...

6 Hebe
July 1847 **
7 Iris Iris (minor planet designation A formal minor planet designation is, in its final form, a number–name combination given to a minor planet A minor planet is an astronomical object in direct orbit In physics, an orbit is the gravit ...
August 1847 **
8 Flora Flora (minor planet designation A formal minor-planet designation is, in its final form, a number–name combination given to a minor planet A minor planet is an astronomical object in direct orbit around the Sun (or more broadly, any star ...
October 1847 **
9 Metis Metis (minor planet designation A formal minor-planet designation is, in its final form, a number–name combination given to a minor planet A minor planet is an astronomical object in direct orbit around the Sun (or more broadly, any st ...
25 April 1848 **
10 Hygiea Hygiea (minor planet designation A formal minor-planet designation is, in its final form, a number–name combination given to a minor planet A minor planet is an astronomical object in direct orbit around the Sun (or more broadly, any st ...
12 April 1849 ''tenth asteroid discovered'' * 100 asteroids by 1868 * 1,000 by 1921 * 10,000 by 1989 * 100,000 by 2005 * 1,000,000 by 2020


Historical methods

Asteroid discovery methods have dramatically improved over the past two centuries. In the last years of the 18th century, Baron
Franz Xaver von Zach Baron Franz Xaver von Zach (''Franz Xaver Freiherr (; male, abbreviated as ), (; his wife, abbreviated as , literally "free lord" or "free lady") and (, his unmarried daughters and maiden aunts) are designations used as titles of nobility ...
organized a group of 24 astronomers to search the sky for the missing planet predicted at about 2.8  from the Sun by the Titius-Bode law, partly because of the discovery, by Sir
William Herschel Sir Frederick William Herschel (; german: Friedrich Wilhelm Herschel; 15 November 1738 – 25 August 1822) was a German-born British astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a spe ...

William Herschel
in 1781, of the planet
Uranus Uranus is the seventh planet from the Sun. Its name is a reference to the Greek god of the sky, Uranus, who, according to Greek mythology Greek mythology is the body of myths originally told by the Ancient Greece, ancient Greeks, and ...

Uranus
at the distance predicted by the law. This task required that hand-drawn sky charts be prepared for all stars in the
zodiac The zodiac is a belt-shaped region of the sky that extends approximately 8° north or south (as measured in celestial latitude) of the ecliptic The ecliptic is the plane (geometry), plane of Earth's orbit around the Sun. From the perspecti ...

zodiac
al band down to an agreed-upon limit of faintness. On subsequent nights, the sky would be charted again and any moving object would, hopefully, be spotted. The expected motion of the missing planet was about 30 seconds of arc per hour, readily discernible by observers. The first object,
Ceres Ceres most commonly refers to: * Ceres (dwarf planet) Ceres (; minor-planet designation: 1 Ceres) is the smallest recognized dwarf planet, the closest dwarf planet to the Sun, and the List of notable asteroids, largest object in the main astero ...
, was not discovered by a member of the group, but rather by accident in 1801 by
Giuseppe Piazzi Giuseppe Piazzi ( , ; 16 July 1746 – 22 July 1826) was an Italian Catholic The Catholic Church, often referred to as the Roman Catholic Church, is the List of Christian denominations by number of members, largest Christian church, wi ...

Giuseppe Piazzi
, director of the observatory of
Palermo Palermo ( , ; scn, Palermu , locally also or ; la, Panormus, from grc, Πάνορμος, Pánormos; older ar, بَلَرْم‎, Balarm) is a city in southern Italy Southern Italy ( it, Sud Italia; nap, 'o Sudde; scn, Italia dû Sud), ...

Palermo
in
Sicily (man) it, Siciliana (woman) , population_note = , population_blank1_title = , population_blank1 = , demographics_type1 = Ethnicity , demographics1_footnotes = , demographi ...

Sicily
. He discovered a new star-like object in
Taurus Taurus is Latin for 'bull' and may refer to: * Taurus (constellation), one of the constellations of the zodiac * Taurus (mythology), one of two Greek mythological characters named Taurus * Taurus (astrology), the astrological sign * ''Bos taurus'' ...
and followed the displacement of this object during several nights. Later that year,
Carl Friedrich Gauss Johann Carl Friedrich Gauss (; german: Gauß ; la, Carolus Fridericus Gauss; 30 April 177723 February 1855) was a German mathematician This is a List of German mathematician A mathematician is someone who uses an extensive knowledge of m ...

Carl Friedrich Gauss
used these observations to calculate the orbit of this unknown object, which was found to be between the planets
Mars Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, being larger than only Mercury (planet), Mercury. In English, Mars carries the name of the Mars (mythology), Roman god of war and is often referred to ...

Mars
and
Jupiter Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but ...

Jupiter
. Piazzi named it after
Ceres Ceres most commonly refers to: * Ceres (dwarf planet) Ceres (; minor-planet designation: 1 Ceres) is the smallest recognized dwarf planet, the closest dwarf planet to the Sun, and the List of notable asteroids, largest object in the main astero ...
, the Roman goddess of agriculture. Three other asteroids (
2 Pallas Pallas (minor-planet designation A formal minor-planet designation is, in its final form, a number–name combination given to a minor planet (asteroid, centaur (minor planet), centaur, trans-Neptunian object and dwarf planet but not comet) ...

2 Pallas
,
3 Juno 3 Juno is a large asteroid in the asteroid belt. Juno was the third asteroid discovered, in 1804, by German astronomer Karl Ludwig Harding, Karl Harding. It is one of the List of exceptional asteroids, twenty largest asteroids and one of the two l ...
, and
4 Vesta Vesta (minor-planet designation A formal minor planet designation is, in its final form, a number–name combination given to a minor planet (asteroid, centaur (minor planet), centaur, trans-Neptunian object and dwarf planet but not comet). S ...
) were discovered over the next few years, with Vesta found in 1807. After eight more years of fruitless searches, most astronomers assumed that there were no more and abandoned any further searches. However,
Karl Ludwig Hencke Karl Ludwig Hencke (8 April 1793 – 21 September 1866) was a German amateur astronomer and discoverer of minor planet A minor planet is an astronomical object in direct orbit In physics, an orbit is the gravitationally curved traje ...

Karl Ludwig Hencke
persisted, and began searching for more asteroids in 1830. Fifteen years later, he found 5 Astraea, the first new asteroid in 38 years. He also found
6 Hebe Hebe (minor planet designation A formal minor-planet designation is, in its final form, a number–name combination given to a minor planet A minor planet is an astronomical object in direct orbit around the Sun (or more broadly, any star ...

6 Hebe
less than two years later. After this, other astronomers joined in the search and at least one new asteroid was discovered every year after that (except the wartime year 1945). Notable asteroid hunters of this early era were , A. de Gasparis, R. Luther, H.M.S. Goldschmidt, J. Chacornac, J. Ferguson, N.R. Pogson, , , C.H.F. Peters, A. Borrelly, , the Henry brothers and A. Charlois. In 1891,
Max Wolf Maximilian Franz Joseph Cornelius Wolf (21 June 1863 – 3 October 1932) was a German astronomer and a pioneer in the field of astrophotography. He was the chairman of astronomy at the University of Heidelberg and director of the Heidelberg-K ...

Max Wolf
pioneered the use of
astrophotography Astrophotography, also known as astronomical imaging, is of s, s, and areas of the . The first photograph of an astronomical object (the ) was taken in 1840, but it was not until the late 19th century that advances in technology allowed for deta ...

astrophotography
to detect asteroids, which appeared as short streaks on long-exposure photographic plates. This dramatically increased the rate of detection compared with earlier visual methods: Wolf alone discovered 248 asteroids, beginning with
323 Brucia Brucia (minor planet designation: 323 Brucia) is a stony Phocaea family, Phocaea asteroid and former Mars-crosser from the inner regions of the asteroid belt, approximately in diameter. It was the first asteroid to be discovered by the use of as ...

323 Brucia
, whereas only slightly more than 300 had been discovered up to that point. It was known that there were many more, but most astronomers did not bother with them, some calling them "vermin of the skies", a phrase variously attributed to E. Suess and . Even a century later, only a few thousand asteroids were identified, numbered and named.


Manual methods of the 1900s and modern reporting

Until 1998, asteroids were discovered by a four-step process. First, a region of the sky was by a wide-field
telescope A telescope is an optical instrument An optical instrument (or "optic" for short) is a device that processes light waves (or photons), either to enhance an image for viewing or to analyze and determine their characteristic properties. Common ...

telescope
, or
astrograph observatory. File:Astrograph in Heidelberg-Königstuhl-2.jpg, The ''Bruce double astrograph'' at the Landessternwarte Heidelberg-Königstuhl observatory. An astrograph (or astrographic camera) is a telescope designed for the sole purpose of as ...
. Pairs of photographs were taken, typically one hour apart. Multiple pairs could be taken over a series of days. Second, the two films or plates of the same region were viewed under a
stereoscope A stereoscope is a device for viewing a stereoscopy, stereoscopic pair of separate images, depicting left-eye and right-eye views of the same scene, as a single three-dimensional image. A typical stereoscope provides each eye with a lens that m ...

stereoscope
. Any body in orbit around the Sun would move slightly between the pair of films. Under the stereoscope, the image of the body would seem to float slightly above the background of stars. Third, once a moving body was identified, its location would be measured precisely using a digitizing microscope. The location would be measured relative to known star locations. These first three steps do not constitute asteroid discovery: the observer has only found an apparition, which gets a
provisional designation Provisional designation in astronomy is the naming convention applied to astronomical objects immediately following their discovery. The provisional designation is usually superseded by a permanent designation once a reliable orbit has been calc ...
, made up of the year of discovery, a letter representing the half-month of discovery, and finally a letter and a number indicating the discovery's sequential number (example: ). The last step of discovery is to send the locations and time of observations to the
Minor Planet Center The Minor Planet Center (MPC) is the official body for observing and reporting on minor planets under the auspices of the International Astronomical Union The International Astronomical Union (IAU; french: link=yes, Union astronomique interna ...
, where computer programs determine whether an apparition ties together earlier apparitions into a single orbit. If so, the object receives a catalogue number and the observer of the first apparition with a calculated orbit is declared the discoverer, and granted the honor of naming the object subject to the approval of the
International Astronomical Union The International Astronomical Union (IAU; french: link=yes, Union astronomique internationale, UAI) is a Non-governmental organization, nongovernmental organisation with the objective of advancing astronomy in all aspects, including promoting ...
.


Computerized methods

There is increasing interest in identifying asteroids whose orbits cross
Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remaining 70.8% is Water distribution on Earth, covered wi ...

Earth
's, and that could, given enough time, collide with Earth ''(see
Earth-crosser asteroids File:Trajectory of 2004 FH in the Earth–Moon system.svg, thumbnail, 280px, Close approach trajectory of 2004 FH in the Earth–Moon system An Earth-crosser is a Near-Earth object#Near-Earth asteroids, near-Earth asteroid whose orbit crosses th ...
)''. The three most important groups of
near-Earth asteroid A near-Earth object (NEO) is any small Solar System body A small Solar System body (SSSB) is an object in the Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regar ...
s are the
Apollos Apollos ( grc-gre, Ἀπολλώς) was a Christianity in the 1st century, 1st-century History of the Jews in Alexandria, Alexandrian Jewish Jewish Christian, Christian mentioned several times in the New Testament. A contemporary and colleague o ...
, Amors, and
Atens Image:Minor Planets - Aten.svg, 285px, The ''Aten group'' compared to the orbits of the terrestrial planets of the Solar System. The Aten asteroids are a List of minor-planet groups, dynamical group of asteroids whose orbits bring them into Near- ...
. Various asteroid deflection strategies have been proposed, as early as the 1960s. The near-Earth asteroid
433 Eros Eros (minor planet designation: 433 Eros), Provisional designation in astronomy, provisional designation , is a S-type asteroid, stony and elongated asteroid of the Amor asteroid, Amor group and the first discovered and second-largest near-Eart ...
had been discovered as long ago as 1898, and the 1930s brought a flurry of similar objects. In order of discovery, these were:
1221 Amor 1221 Amor is an asteroid An asteroid is a minor planet of the Solar System#Inner solar system, inner Solar System. Historically, these terms have been applied to any astronomical object orbiting the Sun that did not resolve into a disc in a ...
,
1862 Apollo 1862 Apollo is a stony asteroid, approximately 1.5 kilometers in diameter, classified as a near-Earth object (NEO). It was discovered by German astronomer Karl Reinmuth at Landessternwarte Heidelberg-Königstuhl, Heidelberg Observatory on 24 Apr ...
,
2101 Adonis 2101 Adonis, provisional designation , is an extremely eccentric asteroid, classified as potentially hazardous asteroid and near-Earth object of the Apollo asteroid, Apollo group, approximately 1 kilometer in diameter. Discovered by Eugène Delpor ...
, and finally
69230 Hermes 69230 Hermes is a sub-kilometer sized asteroid An asteroid is a minor planet of the Solar System#Inner solar system, inner Solar System. Historically, these terms have been applied to any astronomical object orbiting the Sun that did not res ...
, which approached within 0.005  of
Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remaining 70.8% is Water distribution on Earth, covered wi ...

Earth
in 1937. Astronomers began to realize the possibilities of Earth impact. Two events in later decades increased the alarm: the increasing acceptance of the
Alvarez hypothesis The Alvarez hypothesis posits that the mass extinction An extinction event (also known as a mass extinction or biotic crisis) is a widespread and rapid decrease in the biodiversity Biodiversity is the biological variety and Genetic varia ...
that an
impact event An impact event is a between s causing measurable effects. Impact events have physical consequences and have been found to regularly occur in s, though the most frequent involve s, s or s and have minimal effect. When large objects impact s such ...

impact event
resulted in the Cretaceous–Paleogene extinction, and the 1994 observation of Comet Shoemaker-Levy 9 crashing into
Jupiter Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but ...

Jupiter
. The U.S. military also declassified the information that its
military satellite A military satellite is an artificial satellite alt=, A full-size model of the Earth observation satellite ERS 2 ">ERS_2.html" ;"title="Earth observation satellite ERS 2">Earth observation satellite ERS 2 In the context of spaceflight, a ...
s, built to detect nuclear explosions, had detected hundreds of upper-atmosphere impacts by objects ranging from one to ten meters across. All these considerations helped spur the launch of highly efficient surveys that consist of charge-coupled device (Charge-coupled device, CCD) cameras and computers directly connected to telescopes. , it was estimated that 89% to 96% of near-Earth asteroids one kilometer or larger in diameter had been discovered. A list of teams using such systems includes: * Lincoln Near-Earth Asteroid Research (LINEAR) * Near-Earth Asteroid Tracking (NEAT) * Spacewatch * LONEOS, Lowell Observatory Near-Earth-Object Search (LONEOS) * Catalina Sky Survey (CSS) * Pan-STARRS * NEOWISE * Asteroid Terrestrial-impact Last Alert System (ATLAS) * Campo Imperatore Near-Earth Object Survey (CINEOS) * Japanese Spaceguard Association * Asiago-DLR Asteroid Survey (ADAS) , the LINEAR system alone has discovered 147,132 asteroids. Among all the surveys, 19,266 near-Earth asteroids have been discovered including almost 900 more than in diameter.


Terminology

file:Ceres and Vesta, Moon size comparison.jpg, The largest asteroid in the previous image, 4 Vesta, Vesta (left), with
Ceres Ceres most commonly refers to: * Ceres (dwarf planet) Ceres (; minor-planet designation: 1 Ceres) is the smallest recognized dwarf planet, the closest dwarf planet to the Sun, and the List of notable asteroids, largest object in the main astero ...
(center) and the Moon (right) shown to scale. Traditionally, small bodies orbiting the Sun were classified as
comet A comet is an icy, small Solar System body A small Solar System body (SSSB) is an object in the Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astr ...

comet
s, asteroids, or
meteoroid A meteoroid () is a small rocky or metallic body in . Meteoroids are significantly smaller than s, and range in size from small grains to one-meter-wide objects. Objects smaller than this are classified as or . Most are fragments from s or as ...
s, with anything smaller than one meter across being called a meteoroid. Beech and Steel's 1995 paper proposed a meteoroid definition including size limits. The term "asteroid", from the Greek word for "star-like", never had a formal definition, with the broader term
minor planet A minor planet is an astronomical object in direct orbit around the Sun (or more broadly, any star with a planetary system) that is neither a planet nor exclusively classified as a comet. Before 2006, the International Astronomical Union (IAU) ...
being preferred by the
International Astronomical Union The International Astronomical Union (IAU; french: link=yes, Union astronomique internationale, UAI) is a Non-governmental organization, nongovernmental organisation with the objective of advancing astronomy in all aspects, including promoting ...
. However, following the discovery of asteroids below ten meters in size, Rubin and Grossman's 2010 paper revised the previous definition of meteoroid to objects between 10 micrometre, µm and 1 meter in size in order to maintain the distinction between asteroids and meteoroids. The smallest asteroids discovered (based on Absolute magnitude#Solar System bodies (H), absolute magnitude ''H'') are with and with both with an estimated size of about 1 meter. In 2006, the term "small Solar System body" was also introduced to cover both most minor planets and comets. Other languages prefer "planetoid" (Greek for "planet-like"), and this term is occasionally used in English especially for larger minor planets such as the
dwarf planet A dwarf planet is a small planetary-mass object that is in direct orbit of the Sun – something smaller than any of the eight classical planets, but still a world in its own right. The prototypical dwarf planet is Pluto. The interest of d ...
s as well as an alternative for asteroids since they are not star-like. The word "
planetesimal Planetesimals are solid objects thought to exist in protoplanetary disks and debris disks. Per the Chamberlin–Moulton planetesimal hypothesis, they are believed to form out of cosmic dust grains. Believed to have formed in the Solar System ab ...
" has a similar meaning, but refers specifically to the small building blocks of the planets that existed when the Solar System was forming. The term "planetule" was coined by the geologist William Daniel Conybeare to describe minor planets, but is not in common use. The three largest objects in the asteroid belt,
Ceres Ceres most commonly refers to: * Ceres (dwarf planet) Ceres (; minor-planet designation: 1 Ceres) is the smallest recognized dwarf planet, the closest dwarf planet to the Sun, and the List of notable asteroids, largest object in the main astero ...
, 2 Pallas, Pallas, and 4 Vesta, Vesta, grew to the stage of protoplanets. Ceres is a
dwarf planet A dwarf planet is a small planetary-mass object that is in direct orbit of the Sun – something smaller than any of the eight classical planets, but still a world in its own right. The prototypical dwarf planet is Pluto. The interest of d ...
, the only one in the inner Solar System. When found, asteroids were seen as a class of objects distinct from comets, and there was no unified term for the two until "small Solar System body" was coined in 2006. The main difference between an asteroid and a comet is that a comet shows a coma due to Outgassing, sublimation of near-surface ices by solar radiation. A few objects have ended up being dual-listed because they were first classified as minor planets but later showed evidence of cometary activity. Conversely, some (perhaps all) comets are eventually depleted of their surface volatiles, volatile ices and become asteroid-like. A further distinction is that comets typically have more eccentric orbits than most asteroids; most "asteroids" with notably eccentric orbits are probably dormant or extinct comets. For almost two centuries, from the discovery of
Ceres Ceres most commonly refers to: * Ceres (dwarf planet) Ceres (; minor-planet designation: 1 Ceres) is the smallest recognized dwarf planet, the closest dwarf planet to the Sun, and the List of notable asteroids, largest object in the main astero ...
in 1801 until the discovery of the first centaur (minor planet), centaur, 2060 Chiron, Chiron in 1977, all known asteroids spent most of their time at or within the orbit of Jupiter, though a few such as 944 Hidalgo, Hidalgo ventured far beyond Jupiter for part of their orbit. Those located between the orbits of Mars and Jupiter were known for many years simply as The Asteroids. When astronomers started finding more small bodies that permanently resided further out than Jupiter, now called centaur (minor planet), centaurs, they numbered them among the traditional asteroids, though there was debate over whether they should be considered asteroids or as a new type of object. Then, when the first trans-Neptunian object (other than Pluto), 15760 Albion, Albion, was discovered in 1992, and especially when large numbers of similar objects started turning up, new terms were invented to sidestep the issue: Kuiper belt, Kuiper-belt object, trans-Neptunian object, scattered-disc object, and so on. These inhabit the cold outer reaches of the Solar System where ices remain solid and comet-like bodies are not expected to exhibit much cometary activity; if centaurs or trans-Neptunian objects were to venture close to the Sun, their volatile ices would sublimate, and traditional approaches would classify them as comets and not asteroids. The innermost of these are the Kuiper belt, Kuiper-belt objects, called "objects" partly to avoid the need to classify them as asteroids or comets. They are thought to be predominantly comet-like in composition, though some may be more akin to asteroids. Furthermore, most do not have the highly eccentric orbits associated with comets, and the ones so far discovered are larger than traditional Comet nucleus, comet nuclei. (The much more distant Oort cloud is hypothesized to be the main reservoir of dormant comets.) Other recent observations, such as the analysis of the cometary dust collected by the Stardust (spacecraft), ''Stardust'' probe, are increasingly blurring the distinction between comets and asteroids, suggesting "a continuum between asteroids and comets" rather than a sharp dividing line. The minor planets beyond Jupiter's orbit are sometimes also called "asteroids", especially in popular presentations. However, it is becoming increasingly common for the term "asteroid" to be restricted to minor planets of the inner Solar System. Therefore, this article will restrict itself for the most part to the classical asteroids: objects of the
asteroid belt The asteroid belt is a torus-shaped region in the Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizin ...

asteroid belt
,
Jupiter trojan The Jupiter trojans, commonly called trojan asteroids or simply trojans, are a large group of asteroid An asteroid is a minor planet of the Solar System#Inner solar system, inner Solar System. Historically, these terms have been applied to ...
s, and
near-Earth object A near-Earth object (NEO) is any small Solar System body A small Solar System body (SSSB) is an object in the Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body rega ...
s. When the IAU introduced the class Small Solar System body, small Solar System bodies in 2006 to include most objects previously classified as minor planets and comets, they created the class of
dwarf planet A dwarf planet is a small planetary-mass object that is in direct orbit of the Sun – something smaller than any of the eight classical planets, but still a world in its own right. The prototypical dwarf planet is Pluto. The interest of d ...
s for the largest minor planets – those that have enough mass to have become ellipsoidal under their own gravity. According to the IAU, "the term 'minor planet' may still be used, but generally, the term 'Small Solar System Body' will be preferred." Currently only the largest object in the asteroid belt,
Ceres Ceres most commonly refers to: * Ceres (dwarf planet) Ceres (; minor-planet designation: 1 Ceres) is the smallest recognized dwarf planet, the closest dwarf planet to the Sun, and the List of notable asteroids, largest object in the main astero ...
, at about across, has been placed in the dwarf planet category.


Formation

It is thought that
planetesimal Planetesimals are solid objects thought to exist in protoplanetary disks and debris disks. Per the Chamberlin–Moulton planetesimal hypothesis, they are believed to form out of cosmic dust grains. Believed to have formed in the Solar System ab ...
s in the asteroid belt evolved much like the rest of the
solar nebula The formation and evolution of the Solar System The Solar SystemCapitalization Capitalization ( North American English) or capitalisation ( British English) is writing a word with its first letter as a capital letter (uppercase letter) ...
until Jupiter neared its current mass, at which point excitation from orbital resonances with Jupiter ejected over 99% of planetesimals in the belt. Simulations and a discontinuity in spin rate and spectral properties suggest that asteroids larger than approximately in diameter Accretion (astrophysics), accreted during that early era, whereas smaller bodies are fragments from collisions between asteroids during or after the Jovian disruption. Ceres and Vesta grew large enough to melt and Planetary differentiation, differentiate, with heavy metallic elements sinking to the core, leaving rocky minerals in the crust. In the Nice model, many Kuiper belt, Kuiper-belt objects are captured in the outer asteroid belt, at distances greater than 2.6 AU. Most were later ejected by Jupiter, but those that remained may be the D-type asteroids, and possibly include Ceres.


Distribution within the Solar System

Various dynamical groups of asteroids have been discovered orbiting in the inner Solar System. Their orbits are perturbed by the gravity of other bodies in the Solar System and by the Yarkovsky effect. Significant populations include:


Asteroid belt

The majority of known asteroids orbit within the asteroid belt between the orbits of
Mars Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, being larger than only Mercury (planet), Mercury. In English, Mars carries the name of the Mars (mythology), Roman god of war and is often referred to ...

Mars
and
Jupiter Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but ...

Jupiter
, generally in relatively low-orbital eccentricity, eccentricity (i.e. not very elongated) orbits. This belt is now estimated to contain between 1.1 and 1.9 million asteroids larger than in diameter, and millions of smaller ones. These asteroids may be remnants of the protoplanetary disk, and in this region the accretion (astrophysics), accretion of
planetesimal Planetesimals are solid objects thought to exist in protoplanetary disks and debris disks. Per the Chamberlin–Moulton planetesimal hypothesis, they are believed to form out of cosmic dust grains. Believed to have formed in the Solar System ab ...
s into planets during the formative period of the Solar System was prevented by large gravitational perturbations by
Jupiter Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but ...

Jupiter
.


Trojans

Trojan (astronomy), Trojans are populations that share an orbit with a larger planet or moon, but do not collide with it because they orbit in one of the two Lagrangian points of stability, Trojan points, L4 and L5, which lie 60° ahead of and behind the larger body. The most significant population of trojans are the
Jupiter trojan The Jupiter trojans, commonly called trojan asteroids or simply trojans, are a large group of asteroid An asteroid is a minor planet of the Solar System#Inner solar system, inner Solar System. Historically, these terms have been applied to ...
s. Although fewer Jupiter trojans have been discovered (), it is thought that they are as numerous as the asteroids in the asteroid belt. Trojans have been found in the orbits of other planets, including Venus trojan, Venus, Earth trojan, Earth, Mars trojan, Mars, Uranus trojan, Uranus, and Neptune trojan, Neptune.


Near-Earth asteroids

File:Asteroids-KnownNearEarthObjects-Animation-UpTo20180101.gif, upright=1.7, Known Near-Earth objects as of January 2018Video (0:55; 23 July 2018)
File:SmallAsteroidImpacts-Frequency-Bolide-20141114.jpg, upright=1.7, Frequency of bolides, small asteroids roughly 1 to 20 meters in diameter impacting Earth's atmosphere Near-Earth asteroids, or NEAs, are asteroids that have orbits that pass close to that of Earth. Asteroids that actually cross Earth's orbital path are known as ''Earth-crossers''. , 14,464 near-Earth asteroids are known and the number over one kilometer in diameter is estimated to be 900–1,000.


Characteristics


Size distribution

File:Asteroids by size and number.svg, upright=1.5, The asteroids of the Solar System, categorized by size and number Asteroids vary greatly in size, from almost for the largest down to rocks just 1 meter across. The three largest are very much like miniature planets: they are roughly spherical, have at least partly differentiated interiors, and are thought to be surviving protoplanets. The vast majority, however, are much smaller and are irregularly shaped; they are thought to be either battered
planetesimal Planetesimals are solid objects thought to exist in protoplanetary disks and debris disks. Per the Chamberlin–Moulton planetesimal hypothesis, they are believed to form out of cosmic dust grains. Believed to have formed in the Solar System ab ...
s or fragments of larger bodies. The
dwarf planet A dwarf planet is a small planetary-mass object that is in direct orbit of the Sun – something smaller than any of the eight classical planets, but still a world in its own right. The prototypical dwarf planet is Pluto. The interest of d ...
Ceres Ceres most commonly refers to: * Ceres (dwarf planet) Ceres (; minor-planet designation: 1 Ceres) is the smallest recognized dwarf planet, the closest dwarf planet to the Sun, and the List of notable asteroids, largest object in the main astero ...
is by far the largest asteroid, with a diameter of . The next largest are
4 Vesta Vesta (minor-planet designation A formal minor planet designation is, in its final form, a number–name combination given to a minor planet (asteroid, centaur (minor planet), centaur, trans-Neptunian object and dwarf planet but not comet). S ...
and
2 Pallas Pallas (minor-planet designation A formal minor-planet designation is, in its final form, a number–name combination given to a minor planet (asteroid, centaur (minor planet), centaur, trans-Neptunian object and dwarf planet but not comet) ...

2 Pallas
, both with diameters of just over . Vesta is the only main-belt asteroid that can, on occasion, be visible to the naked eye. On some rare occasions, a near-Earth asteroid may briefly become visible without technical aid; see 99942 Apophis. The mass of all the objects of the
asteroid belt The asteroid belt is a torus-shaped region in the Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizin ...

asteroid belt
, lying between the orbits of
Mars Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, being larger than only Mercury (planet), Mercury. In English, Mars carries the name of the Mars (mythology), Roman god of war and is often referred to ...

Mars
and
Jupiter Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but ...

Jupiter
, is estimated to be in the range of , about 4% of the mass of the Moon. Of this,
Ceres Ceres most commonly refers to: * Ceres (dwarf planet) Ceres (; minor-planet designation: 1 Ceres) is the smallest recognized dwarf planet, the closest dwarf planet to the Sun, and the List of notable asteroids, largest object in the main astero ...
comprises , about a third of the total. Adding in the next three most massive objects, 4 Vesta, Vesta (9%), 2 Pallas, Pallas (7%), and 10 Hygiea, Hygiea (3%), brings this figure up to half, whereas the three most-massive asteroids after that, 511 Davida (1.2%), 704 Interamnia (1.0%), and 52 Europa (0.9%), constitute only another 3%. The number of asteroids increases rapidly as their individual masses decrease. The number of asteroids decreases markedly with size. Although this generally follows a power law, there are 'bumps' at and , where more asteroids than expected from a logarithmic distribution are found.


Largest asteroids

File:The Four Largest Asteroids.jpg, upright=1.35, The four largest asteroids: 1 Ceres, 4 Vesta, 2 Pallas, and 10 Hygiea Although their location in the asteroid belt excludes them from planet status, the three largest objects,
Ceres Ceres most commonly refers to: * Ceres (dwarf planet) Ceres (; minor-planet designation: 1 Ceres) is the smallest recognized dwarf planet, the closest dwarf planet to the Sun, and the List of notable asteroids, largest object in the main astero ...
, 4 Vesta, Vesta, and 2 Pallas, Pallas, are intact protoplanets that share many characteristics common to planets, and are atypical compared to the majority of irregularly shaped asteroids. The fourth-largest asteroid, 10 Hygiea, Hygiea, appears nearly spherical although it may have an undifferentiated interior, like the majority of asteroids. Between them, the four largest asteroids constitute half the mass of the asteroid belt. Ceres is the only asteroid with a fully ellipsoidal shape and hence the only one that is a
dwarf planet A dwarf planet is a small planetary-mass object that is in direct orbit of the Sun – something smaller than any of the eight classical planets, but still a world in its own right. The prototypical dwarf planet is Pluto. The interest of d ...
. It has a much higher Absolute magnitude#Solar System bodies (H), absolute magnitude than the other asteroids, of around 3.32, and may possess a surface layer of ice. Like the planets, Ceres is differentiated: it has a crust, a mantle and a core. No meteorites from Ceres have been found on Earth. Vesta, too, has a differentiated interior, though it formed inside the Solar System's Frost line (astrophysics), frost line, and so is devoid of water; its composition is mainly of basaltic rock with minerals such as olivine. Aside from the large crater at its southern pole, Rheasilvia, Vesta also has an ellipsoidal shape. Vesta is the parent body of the Vestian family and other V-type asteroids, and is the source of the HED meteorites, which constitute 5% of all meteorites on Earth. Pallas is unusual in that, like
Uranus Uranus is the seventh planet from the Sun. Its name is a reference to the Greek god of the sky, Uranus, who, according to Greek mythology Greek mythology is the body of myths originally told by the Ancient Greece, ancient Greeks, and ...

Uranus
, it rotates on its side, with its axis of rotation tilted at high angles to its orbital plane. Its composition is similar to that of Ceres: high in carbon and silicon, and perhaps partially differentiated. Pallas is the parent body of the Palladian family of asteroids. Hygiea is the largest carbonaceous asteroid and, unlike the other largest asteroids, lies relatively close to the plane of the ecliptic. It is the largest member and presumed parent body of the Hygiean family of asteroids. Because there is no sufficiently large crater on the surface to be the source of that family, as there is on Vesta, it is thought that Hygiea may have been completely disrupted in the collision that formed the Hygiean family and recoalesced after losing a bit less than 2% of its mass. Observations taken with the Very Large Telescope's VLT-SPHERE, SPHERE imager in 2017 and 2018, and announced in late 2019, revealed that Hygiea has a nearly spherical shape, which is consistent both with it being in hydrostatic equilibrium (and thus a
dwarf planet A dwarf planet is a small planetary-mass object that is in direct orbit of the Sun – something smaller than any of the eight classical planets, but still a world in its own right. The prototypical dwarf planet is Pluto. The interest of d ...
), or formerly being in hydrostatic equilibrium, or with being disrupted and recoalescing. File:masses of asteroids vs main belt.png, upright=1.75, left, The relative masses of the twelve List of notable asteroids#Largest by mass, largest asteroids known, compared to the remaining mass of the asteroid belt.


Rotation

Measurements of the rotation rates of large asteroids in the asteroid belt show that there is an upper limit. Very few asteroids with a diameter larger than 100 meters have a rotation period smaller than 2.2 hours. For asteroids rotating faster than approximately this rate, the inertial force at the surface is greater than the gravitational force, so any loose surface material would be flung out. However, a solid object should be able to rotate much more rapidly. This suggests that most asteroids with a diameter over 100 meters are rubble piles formed through the accumulation of debris after collisions between asteroids.


Composition

File:Vesta Cratered terrain with hills and ridges.jpg, Cratered terrain on 4 Vesta The physical composition of asteroids is varied and in most cases poorly understood. Ceres appears to be composed of a rocky core covered by an icy mantle, where Vesta is thought to have a nickel-iron core, olivine mantle, and basaltic crust.
10 Hygiea Hygiea (minor planet designation A formal minor-planet designation is, in its final form, a number–name combination given to a minor planet A minor planet is an astronomical object in direct orbit around the Sun (or more broadly, any st ...
, however, which appears to have a uniformly primitive composition of carbonaceous chondrite, is thought to be the largest undifferentiated asteroid. Most of the smaller asteroids are thought to be piles of rubble held together loosely by gravity, though the largest are probably solid. Some asteroids have Asteroid moon, moons or are co-orbiting binary asteroid, binaries: Rubble piles, moons, binaries, and scattered asteroid family, asteroid families are thought to be the results of collisions that disrupted a parent asteroid, or, possibly, a disrupted planet, planet. Asteroids contain traces of amino acids and other organic compounds, and some speculate that asteroid impacts may have seeded the early Earth with the chemicals necessary to initiate life, or may have even brought life itself to Earth ''(also see panspermia)''. In August 2011, a report, based on
NASA The National Aeronautics and Space Administration (NASA; ) is an independent agency A regulatory agency or regulatory authority, is a Public benefit corporation Public-benefit corporation is a term that has different meanings in differen ...

NASA
studies with meteorites found on
Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remaining 70.8% is Water distribution on Earth, covered wi ...

Earth
, was published suggesting DNA and RNA components (adenine, guanine and related organic molecules) may have been formed on asteroids and
comet A comet is an icy, small Solar System body A small Solar System body (SSSB) is an object in the Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astr ...

comet
s in outer space. File:PIA18469-AsteroidCollision-NearStarNGC2547-ID8-2013.jpg, Asteroid collision building planets (artist concept). Composition is calculated from three primary sources: albedo, surface spectrum, and density. The last can only be determined accurately by observing the orbits of moons the asteroid might have. So far, every asteroid with moons has turned out to be a rubble pile, a loose conglomeration of rock and metal that may be half empty space by volume. The investigated asteroids are as large as 280 km in diameter, and include 121 Hermione (268×186×183 km), and 87 Sylvia (384×262×232 km). Only half a dozen asteroids are List of notable asteroids#Largest by diameter, larger than 87 Sylvia, though none of them have moons; however, some smaller asteroids are thought to be more massive, suggesting they may not have been disrupted, and indeed 511 Davida, the same size as Sylvia to within measurement error, is estimated to be two and a half times as massive, though this is highly uncertain. The fact that such large asteroids as Sylvia can be rubble piles, presumably due to disruptive impacts, has important consequences for the formation of the Solar System: Computer simulations of collisions involving solid bodies show them destroying each other as often as merging, but colliding rubble piles are more likely to merge. This means that the cores of the planets could have formed relatively quickly. On 7 October 2009, the presence of ice, water ice was confirmed on the surface of 24 Themis using
NASA The National Aeronautics and Space Administration (NASA; ) is an independent agency A regulatory agency or regulatory authority, is a Public benefit corporation Public-benefit corporation is a term that has different meanings in differen ...

NASA
's Infrared Telescope Facility. The surface of the asteroid appears completely covered in ice. As this ice layer is Sublimation (phase transition), sublimating, it may be getting replenished by a reservoir of ice under the surface. Organic compounds were also detected on the surface. Scientists hypothesize that some of the first water brought to
Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remaining 70.8% is Water distribution on Earth, covered wi ...

Earth
was delivered by asteroid impacts after the collision that produced the Moon. The presence of ice on 24 Themis supports this theory. In October 2013, water was detected on an extrasolar body for the first time, on an asteroid orbiting the white dwarf GD 61. On 22 January 2014, European Space Agency (ESA) scientists reported the detection, for the first definitive time, of water vapor on
Ceres Ceres most commonly refers to: * Ceres (dwarf planet) Ceres (; minor-planet designation: 1 Ceres) is the smallest recognized dwarf planet, the closest dwarf planet to the Sun, and the List of notable asteroids, largest object in the main astero ...
, the largest object in the asteroid belt. The detection was made by using the Far-infrared astronomy, far-infrared abilities of the Herschel Space Observatory. The finding is unexpected because comets, not asteroids, are typically considered to "sprout jets and plumes". According to one of the scientists, "The lines are becoming more and more blurred between comets and asteroids." In May 2016, significant asteroid data arising from the Wide-field Infrared Survey Explorer and Wide-field Infrared Survey Explorer#NEOWISE, NEOWISE missions have been questioned. Although the early original criticism had not undergone peer review, a more recent peer-reviewed study was subsequently published. In November 2019, scientists reported detecting, for the first time, Sugar, sugar molecules, including ribose, in meteorites, suggesting that chemical processes on asteroids can produce some fundamentally essential bio-ingredients important to life, and supporting the notion of an RNA world prior to a DNA-based Abiogenesis, origin of life on Earth, and possibly, as well, the notion of panspermia. Acfer 049, a meteorite discovered in Algeria in 1990, was shown in 2019 to have ice fossils inside it – the first direct evidence of water ice in the composition of asteroids.


Surface features

Most asteroids outside the "List of exceptional asteroids#Largest by mass, big four" (Ceres, Pallas, Vesta, and Hygiea) are likely to be broadly similar in appearance, if irregular in shape. 50 km (31 mi) 253 Mathilde is a rubble pile saturated with craters with diameters the size of the asteroid's radius, and Earth-based observations of 300 km (186 mi) 511 Davida, one of the largest asteroids after the big four, reveal a similarly angular profile, suggesting it is also saturated with radius-size craters. Medium-sized asteroids such as Mathilde and 243 Ida that have been observed up close also reveal a deep regolith covering the surface. Of the big four, Pallas and Hygiea are practically unknown. Vesta has compression fractures encircling a radius-size crater at its south pole but is otherwise a spheroid. Ceres seems quite different in the glimpses Hubble has provided, with surface features that are unlikely to be due to simple craters and impact basins, but details will be expanded with the ''Dawn (spacecraft), Dawn spacecraft'', which entered Ceres orbit on 6 March 2015.


Color

Asteroids become darker and redder with age due to space weathering. However evidence suggests most of the color change occurs rapidly, in the first hundred thousand years, limiting the usefulness of spectral measurement for determining the age of asteroids.


Classification

File:Kirkwood-20060509.png, Showing Kirkwood gaps, by showing positions based on their semi-major axis Asteroids are commonly categorized according to two criteria: the characteristics of their orbits, and features of their reflectance visible spectrum, spectrum.


Orbital classification

Many asteroids have been placed in groups and families based on their orbital characteristics. Apart from the broadest divisions, it is customary to name a group of asteroids after the first member of that group to be discovered. Groups are relatively loose dynamical associations, whereas families are tighter and result from the catastrophic break-up of a large parent asteroid sometime in the past. Families are more common and easier to identify within the main asteroid belt, but several small families have been reported among the
Jupiter trojan The Jupiter trojans, commonly called trojan asteroids or simply trojans, are a large group of asteroid An asteroid is a minor planet of the Solar System#Inner solar system, inner Solar System. Historically, these terms have been applied to ...
s. Main belt families were first recognized by Kiyotsugu Hirayama in 1918 and are often called Hirayama families in his honor. About 30–35% of the bodies in the asteroid belt belong to dynamical families each thought to have a common origin in a past collision between asteroids. A family has also been associated with the plutoid
dwarf planet A dwarf planet is a small planetary-mass object that is in direct orbit of the Sun – something smaller than any of the eight classical planets, but still a world in its own right. The prototypical dwarf planet is Pluto. The interest of d ...
.


Quasi-satellites and horseshoe objects

Some asteroids have unusual horseshoe orbits that are co-orbital with
Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remaining 70.8% is Water distribution on Earth, covered wi ...

Earth
or some other planet. Examples are 3753 Cruithne and . The first instance of this type of orbital arrangement was discovered between Saturn's moons Epimetheus (moon), Epimetheus and Janus (moon), Janus. Sometimes these horseshoe objects temporarily become quasi-satellites for a few decades or a few hundred years, before returning to their earlier status. Both Earth and Venus are known to have quasi-satellites. Such objects, if associated with Earth or Venus or even hypothetically Mercury (planet), Mercury, are a special class of Aten asteroids. However, such objects could be associated with outer planets as well.


Spectral classification

File:433eros.jpg, This picture of
433 Eros Eros (minor planet designation: 433 Eros), Provisional designation in astronomy, provisional designation , is a S-type asteroid, stony and elongated asteroid of the Amor asteroid, Amor group and the first discovered and second-largest near-Eart ...
shows the view looking from one end of the asteroid across the gouge on its underside and toward the opposite end. Features as small as across can be seen. In 1975, an asteroid Taxonomy (general), taxonomic system based on color, albedo, and spectral line, spectral shape was developed by Clark R. Chapman, Chapman, David Morrison (astrophysicist), Morrison, and Ben Zellner, Zellner. These properties are thought to correspond to the composition of the asteroid's surface material. The original classification system had three categories: C-type asteroid, C-types for dark carbonaceous objects (75% of known asteroids), S-type asteroid, S-types for stony (silicaceous) objects (17% of known asteroids) and U for those that did not fit into either C or S. This classification has since been expanded to include many other asteroid types. The number of types continues to grow as more asteroids are studied. The two most widely used taxonomies now used are the Tholen classification and SMASS classification. The former was proposed in 1984 by David J. Tholen, and was based on data collected from an eight-color asteroid survey performed in the 1980s. This resulted in 14 asteroid categories. In 2002, the Small Main-Belt Asteroid Spectroscopic Survey resulted in a modified version of the Tholen taxonomy with 24 different types. Both systems have three broad categories of C, S, and X asteroids, where X consists of mostly metallic asteroids, such as the M-type. There are also several smaller classes. The proportion of known asteroids falling into the various spectral types does not necessarily reflect the proportion of all asteroids that are of that type; some types are easier to detect than others, biasing the totals.


Problems

Originally, spectral designations were based on inferences of an asteroid's composition. However, the correspondence between spectral class and composition is not always very good, and a variety of classifications are in use. This has led to significant confusion. Although asteroids of different spectral classifications are likely to be composed of different materials, there are no assurances that asteroids within the same taxonomic class are composed of the same (or similar) materials.


Naming

File:Asteroid20130318-full.jpg, 2013 EC, shown here in radar images, has a provisional designation A newly discovered asteroid is given a Provisional designation in astronomy, provisional designation (such as ) consisting of the year of discovery and an alphanumeric code indicating the half-month of discovery and the sequence within that half-month. Once an asteroid's orbit has been confirmed, it is given a number, and later may also be given a name (e.g. ). The formal naming convention uses parentheses around the number – e.g. (433) Eros – but dropping the parentheses is quite common. Informally, it is common to drop the number altogether, or to drop it after the first mention when a name is repeated in running text. In addition, names can be proposed by the asteroid's discoverer, within guidelines established by the International Astronomical Union.


Symbols

The first asteroids to be discovered were assigned iconic symbols like the ones traditionally used to designate the planets. By 1855 there were two dozen asteroid symbols, which often occurred in multiple variants. In 1851, after the fifteenth asteroid (15 Eunomia, Eunomia) had been discovered, Johann Franz Encke made a major change in the upcoming 1854 edition of the ''Berliner Astronomisches Jahrbuch'' (BAJ, ''Berlin Astronomical Yearbook''). He introduced a disk (circle), a traditional symbol for a star, as the generic symbol for an asteroid. The circle was then numbered in order of discovery to indicate a specific asteroid (although he assigned ① to the fifth, 5 Astraea, Astraea, while continuing to designate the first four only with their existing iconic symbols). The numbered-circle convention was quickly adopted by astronomers, and the next asteroid to be discovered (16 Psyche, in 1852) was the first to be designated in that way at the time of its discovery. However, Psyche was given an iconic symbol as well, as were a few other asteroids discovered over the next few years (see chart above). 20 Massalia was the first asteroid that was not assigned an iconic symbol, and no iconic symbols were created after the 1855 discovery of 37 Fides. That year Astraea's number was increased to ⑤, but the first four asteroids, Ceres to Vesta, were not listed by their numbers until the 1867 edition. The circle was soon abbreviated to a pair of parentheses, which were easier to typeset and sometimes omitted altogether over the next few decades, leading to the modern convention.


Exploration

File:Eros rotation Dec. 3-4 2000.gif, Eros as seen by visiting spacecraft Until the age of Spaceflight, space travel, objects in the asteroid belt were merely pinpricks of light in even the largest telescopes and their shapes and terrain remained a mystery. The best modern ground-based telescopes and the Earth-orbiting Hubble Space Telescope can resolve a small amount of detail on the surfaces of the largest asteroids, but even these mostly remain little more than fuzzy blobs. Limited information about the shapes and compositions of asteroids can be inferred from their light curves (their variation in brightness as they rotate) and their spectral properties, and asteroid sizes can be estimated by timing the lengths of star occultations (when an asteroid passes directly in front of a star). Radar imaging can yield good information about asteroid shapes and orbital and rotational parameters, especially for near-Earth asteroids. In terms of delta-v and propellant requirements, NEOs are more easily accessible than the Moon. The first close-up photographs of asteroid-like objects were taken in 1971, when the ''Mariner 9'' probe imaged Phobos (moon), Phobos and Deimos (moon), Deimos, the two small moons of
Mars Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, being larger than only Mercury (planet), Mercury. In English, Mars carries the name of the Mars (mythology), Roman god of war and is often referred to ...

Mars
, which are probably captured asteroids. These images revealed the irregular, potato-like shapes of most asteroids, as did later images from the Voyager program, Voyager probes of the small moons of the gas giants. The first true asteroid to be photographed in close-up was 951 Gaspra in 1991, followed in 1993 by 243 Ida and its moon Dactyl (asteroid), Dactyl, all of which were imaged by the Galileo (spacecraft), ''Galileo'' probe en route to
Jupiter Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but ...

Jupiter
. The first dedicated asteroid probe was ''NEAR Shoemaker'', which photographed 253 Mathilde in 1997, before entering into orbit around
433 Eros Eros (minor planet designation: 433 Eros), Provisional designation in astronomy, provisional designation , is a S-type asteroid, stony and elongated asteroid of the Amor asteroid, Amor group and the first discovered and second-largest near-Eart ...
, finally landing on its surface in 2001. Other asteroids briefly visited by spacecraft en route to other destinations include 9969 Braille (by ''Deep Space 1'' in 1999), and 5535 Annefrank (by ''Stardust (spacecraft), Stardust'' in 2002). From September to November 2005, the Japanese ''Hayabusa (spacecraft), Hayabusa'' probe studied 25143 Itokawa in detail and was plagued with difficulties, but Sample return mission, returned samples of its surface to Earth on 13 June 2010. The European Rosetta (spacecraft), ''Rosetta'' probe (launched in 2004) flew by 2867 Šteins in 2008 and 21 Lutetia, the third-largest asteroid visited to date, in 2010. In September 2007,
NASA The National Aeronautics and Space Administration (NASA; ) is an independent agency A regulatory agency or regulatory authority, is a Public benefit corporation Public-benefit corporation is a term that has different meanings in differen ...

NASA
launched the Dawn (spacecraft), ''Dawn'' spacecraft, which orbited
4 Vesta Vesta (minor-planet designation A formal minor planet designation is, in its final form, a number–name combination given to a minor planet (asteroid, centaur (minor planet), centaur, trans-Neptunian object and dwarf planet but not comet). S ...
from July 2011 to September 2012, and has been orbiting the dwarf planet Ceres (dwarf planet), 1 Ceres since 2015. 4 Vesta is the second-largest asteroid visited to date. On 13 December 2012, China's lunar orbiter ''Chang'e 2'' flew within of the asteroid 4179 Toutatis on an extended mission. The Japan Aerospace Exploration Agency (JAXA) launched the ''Hayabusa2'' probe in December 2014, and plans to return samples from 162173 Ryugu in December 2020. In June 2018, the US
National Science and Technology Council The National Science and Technology Council (NSTC) is a council in the Executive Branch The executive is the branch of government exercising authority in and holding Moral responsibility, responsibility for the governance of a State (polity), s ...
warned that America is unprepared for an Asteroid impact avoidance, asteroid impact event, and has developed and released the ''"National Near-Earth Object Preparedness Strategy Action Plan"'' to better prepare. File:Bennu 330km 181029.jpg, 100px, left, Bennu In September 2016, NASA launched the OSIRIS-REx sample return mission to asteroid 101955 Bennu, which it reached in December 2018. , the probe is in orbit around the asteroid.


Planned and future missions

File:Lucy--mission-13--v3.png, Planned Lucy spacecraft In early 2013, NASA announced the planning stages of a mission to capture a near-Earth asteroid and move it into lunar orbit where it could possibly be visited by astronauts and later impacted into the Moon. On 19 June 2014, NASA reported that asteroid 2011 MD was a prime candidate for capture by a robotic mission, perhaps in the early 2020s. It has been suggested that asteroids might be used as a source of materials that may be rare or exhausted on Earth (asteroid mining), or materials for constructing space habitats ''(see Colonization of the asteroids)''. Materials that are heavy and expensive to launch from Earth may someday be mined from asteroids and used for space manufacturing and construction. In the U.S. Discovery program the Psyche (spacecraft), ''Psyche'' spacecraft proposal to 16 Psyche and Lucy (spacecraft), ''Lucy'' spacecraft to
Jupiter trojan The Jupiter trojans, commonly called trojan asteroids or simply trojans, are a large group of asteroid An asteroid is a minor planet of the Solar System#Inner solar system, inner Solar System. Historically, these terms have been applied to ...
s made it to the semi-finalist stage of mission selection. In January 2017, ''Lucy'' and Psyche (spacecraft), ''Psyche'' mission were both selected as NASA's Discovery Program missions 13 and 14 respectively. Location of Ceres (within asteroid belt) compared to other bodies of the Solar System


Fiction

Asteroids and the asteroid belt are a staple of science fiction stories. Asteroids play several potential roles in science fiction: as places human beings might colonize, resources for extracting minerals, hazards encountered by spacecraft traveling between two other points, and as a threat to life on Earth or other inhabited planets, dwarf planets, and natural satellites by potential impact.


Gallery

File:951 Gaspra.jpg, 951 Gaspra is the first asteroid to be imaged in close-up, imaged by ''Galileo (spacecraft), Galileo'' on (enhanced color) File:PIA02475 Eros' Bland Butterscotch Colors.jpg, Several views of 433 Eros in natural color, imaged by ''NEAR'' on File:Dawn-image-070911.jpg, Vesta imaged by ''Dawn (spacecraft), Dawn'' on File:Ceres processed.jpg,
Ceres Ceres most commonly refers to: * Ceres (dwarf planet) Ceres (; minor-planet designation: 1 Ceres) is the smallest recognized dwarf planet, the closest dwarf planet to the Sun, and the List of notable asteroids, largest object in the main astero ...
imaged by ''Dawn (spacecraft), Dawn'' on


See also

* Active asteroid * Amor asteroid * Apollo asteroid *
Asteroid Day __NOTOC__ Asteroid Day (also known as International Asteroid Day) is an annual global event which is held on the anniversary of the Siberian Tunguska event that took place on June 30, 1908, the most harmful known asteroid An asteroid is a mi ...
* Asteroid impact avoidance * Aten asteroid * Atira asteroid * BOOTES (Burst Observer and Optical Transient Exploring System) * :Asteroid groups and families, Asteroid groups and families * :Asteroids, Asteroids * :Binary asteroids, Binary asteroids * Centaur (minor planet) * Chang'e 2, Chang'e 2 lunar orbiter * Constellation program * Dawn (spacecraft), ''Dawn'' (spacecraft) * Dwarf planet * Impact event * List of asteroid close approaches to Earth * List of exceptional asteroids * List of impact craters on Earth * List of minor planets * List of minor planets named after people * List of minor planets named after places * List of possible impact structures on Earth * Lost minor planet * Marco Polo (spacecraft), ''Marco Polo'' (spacecraft) * Meanings of minor planet names * Mesoplanet * Minor planet * Near-Earth object * NEOShield * NEOSSat (Near Earth Object Surveillance Satellite) Canada's new satellite * ''Pioneer 10'' * Rosetta (spacecraft), ''Rosetta'' (spacecraft) * ʻOumuamua


Notes


References


Further reading

''Further information about asteroids'' * (see Logarithmic scale) * *


External links

* * * * * * * * * * * * {{Authority control Asteroids, Minor planets